(1) Field of the Invention
The present invention relates to systems for judging a malfunction in a sensor, and more particularly to a system and an integrated circuit for judging a malfunction in a sensor used for flow counter such as a water meter.
(2) Description of the Related Art
Generally, a water meter having a sensor for measuring the amount of water used is included in a water pipe. For example, the water meter in each household measures the amount of water used in the household, and water rate according to the measured amount of water is charged to the household.
Thus, it is necessary to measure accurate amount of water in order to charge accurate water rate to the household. When there is a malfunction in the sensor of the water meter, which makes the sensor unable to measure the precise amount of water, it is necessary to detect the malfunction and the manager of the water service needs to be notified immediately.
The technique disclosed in the Japanese Unexamined Patent Application Publication No. 2004-294442 has been proposed for detecting such a sensor malfunction.
FIG. 13 is a block diagram of the malfunction detection system disclosed in the Japanese Unexamined Patent Application Publication No. 2004-294442. The malfunction detection system in FIG. 13 detects malfunctions in the sensor for detecting the speed of engine, and includes a sensor 60, an interface 61, a malfunction detection circuit 62, and an OR circuit 63. The interface 61 includes a differential amplifier 611 and a speed input comparator 612. The malfunction detection circuit 62 includes a current source 621, a filtering circuit 622, and a malfunction comparator 623. The sensor 60 is connected to the interface 61 and the malfunction detection circuit 62 via two lead wires 64.
In the conventional malfunction detection system, bias current is outputted to the sensor 60 from the current source 621 via the lead wires 64. The current source 621 is driven and maintained by offset voltage. The offset voltage varies as necessary so that the bias current can be maintained at a predetermined level. The malfunction comparator 623 detects malfunction by monitoring the offset voltage.
When the sensor 60 is experiencing a malfunction (when the sensor is open or short circuited), the offset voltage increases in order to maintain the bias current at a constant level. Here, the malfunction detection circuit 62 generates a malfunction signal when the offset voltage exceeds a predetermined threshold, assuming that the sensor 60 is malfunctioning.
Note that in the configuration in FIG. 13, there is a possibility that the offset voltage varies depending on the output from the sensor 60 (alternating current signal). In response to this, the filtering circuit 622 is provided to eliminate the output component of the sensor 60. The filtering circuit 622 is a low-pass filter including a resistor and a capacitor.